Medial Medullary Reticular Formation Research Articles

The origin of brainstem afferents of the paramedian pontine reticular formation in the cat

Transcannular microinjections of horseradish peroxidase (HRP) were made into the paramedian pontine reticular formation (PPRF) in adult cats to determine the origin of the principal sources of inputs to this important preoculomotor center for the production of saccadic eye movements. Retrogradely labeled cells were observed in numerous oculomotor-related structures, including the prerubral field (rostral interstitial nucleus of the medial longitudinal fasciculus), nucleus of Darkschewitsh, nucleus of the posterior commissure, deep superior colliculus, supraoculomotor ventral periaqueductal gray, contralateral paramedian pontine reticular formation, pontine raphe and dorsal medial pontine tegmentum medial to the abducens nucleus (purported to contain omnipause neurons), cell group Y, and the perihypoglossal complex (nucleus prepositus hypoglossi). Other sources of afferents to the region included the zona incerta, lateral and medial habenular nuclei, medial hypothalamus, medial mammillary nucleus, nucleus cuneiformis, medial medullary reticular formation, and the medial and lateral cerebellar nuclei. The results are discussed in terms of the potential influence of these nuclei on the control of eye movement.

Characteristics of spinoreticular and spinothalamic neurons with renal input.

Spinoreticular (SRT) and spinothalamic (STT) neurons were studied for responses to renal and somatic stimuli in 34 cats that were anesthetized with alpha-chloralose. SRT cells were antidromically activated from the medial medullary reticular formation near the gigantocellular tegmental field contralateral (35 cells), ipsilateral (15 cells), or both contralateral and ipsilateral (11 cells) to the recording site. Collision tests showed that activation from two electrodes resulted from stimulation of separate axonal branches and not from current spread. Twenty STT cells were activated from the spinothalamic tract just medial to the medial geniculate nucleus. SRT cells were located in laminae I, V, VII, and VIII of the T12-L2 segments. Most cells were located in lamina VII. STT cells were found in laminae I, V, and VII. The axons of 12 SRT cells were located in the ventrolateral or ventral quadrants of the upper cervical spinal cord. Antidromic conduction velocities of SRT cells averaged 48.7 +/- 3.7 m/s. No differences in conduction velocity were found between cells projecting to different reticular sites. In addition conduction velocity did not vary with the type of somatic or renal input. Antidromic conduction velocities of STT cells averaged 46.4 +/- 4.7 m/s. Renal nerve stimulation excited 58 and inhibited 3 SRT cells. All 20 STT cells were excited. Thirty SRT cells were excited only by A-delta input, 26 received both A-delta- and C-fiber inputs, and 2 cells received only C-fiber input. Ten STT cells received A-delta input only and 10 received both A-delta- and C-fiber inputs. All cells with renal input also received somatic input. Thirty-six SRT cells (59%) were classified as high threshold, 12 (20%) as wide dynamic range, and 10 (16%) as deep. Ten STT cells were classified as high threshold and 10 as wide dynamic range. Somatic receptive fields of STT cells were usually simple and invariably included the left flank region, although many of the fields extended to the left hindlimb or abdomen. Eighteen of the 20 were restricted to the ipsilateral side. In contrast, somatic receptive fields of SRT cells were primarily bilateral (71%). While all but two receptive fields included the left flank area, most extended to one or both hindlimbs, the abdomen, or the right flank. Inhibitory receptive fields were found for 33% of the SRT cells and 20% of the STT cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of blood pressure on medial medulla-induced muscle atonia.

Stimulation of the medial medullary reticular formation (MMRF) has long been reported to produce generalized inhibition of skeletal muscle activity. However, several studies have reported that in most cases MMRF stimulation produces only increases in muscle tone. In the present investigation we have found that blood pressure is a critical variable, determining whether MMRF stimulation will produce muscle excitation or inhibition. When mean arterial pressure (MAP) was greater than 80 mmHg but less than 148 mmHg, MMRF stimulation produced muscle antonia. Reductions of blood pressure by pharmacological or mechanical techniques induced a reversal of response to MMRF stimulation; stimulation that produced inhibition in base-line conditions produced excitation after MAP reduction. MAP reductions of as little as 10% could cause the reversal response. In contrast, the EMG reduction to MMRF stimulation was not changed or was augmented when MAP was raised. MMRF induced atonia, and its reversal by blood pressure reduction persisted after bilateral isolation of the carotid sinus combined with vagotomy, and in the 6-hydroxydopamine-treated cat. Spinal transection at the cervicothoracic junction did not block atonia or the reversal response. It is suggested that the reversal is mediated centrally.

Retrograde labeling of neurones in the brain stem following injections of [3H]choline into the forebrain of the rat.

In an attempt to identify cholinergic neurons of the brain stem which project to the forebrain, retrograde labeling of neurons in the brain stem was examined by autoradiography following injections of 20 microCi [3H]choline into the thalamus, hypothalamus, basal forebrain and frontal cortex. After injections into the thalamus, retrogradely labeled neurons were evident within the lateral caudal mesencephalic and dorsolateral oral pontine tegmentum (particularly in the laterodorsal and pedunculopontine tegmental nuclei) and in smaller number within the latero-medial caudal pontine (Reticularis pontis caudalis, Rpc) and medullary (Reticularis gigantocellularis, Rgc) reticular formation. Following [3H]choline injections into the lateral hypothalamus and into the basal forebrain, retrogradely labeled neurons were localized in the dorsolateral caudal midbrain and oral pontine tegmentum and in smaller number in the medial medullary reticular formation (Rgc), as well as in the midbrain, pontine and medullary raphe nuclei. After injections into the anterior medial frontal cortex, a small number of retrogradely labeled cells were found in the brain stem within the laterodorsal tegmental nucleus and the dorsal raphe nucleus. In a parallel immunohistochemical study, choline acetyltransferase (ChAT)-positive neurons were found to be located in most of the regions of the reticular formation where cells were retrogradely labeled from the forebrain following [3H]choline injections. These results suggest that multiple cholinergic neurons within the lateral caudal midbrain and dorsolateral oral pontine tegmentum and a few within the caudal pontine and medullary reticular formation project to the thalamus, hypothalamus and basal forebrain and that a limited number of pontine cholinergic neurons project to the frontal cortex.

Retrograde labeling of neurons in the brain stem following injections of [ 3H]choline into the rat spinal cord

In an attempt to identify cholinergic neurons in the brain stem which project to the spinal cord, [ 3H]choline (100,20, 10, 5 or 1 μ Ci) was injected into the upper cervical spinal cord in 55 rats. The animals were killed 20 h later and the brains processed for autoradiography of diffusible substances. At all doses of [ 3H]choline, cells were consistently, retrogradely labeled in the medial medullary reticular formation, the lateral vestibular nucleus, the dorsolateral pontine tegmentum and the red nucleus. The retrogradely labeled cells were found to be moderately to darkly stained for acetylcholinesterase. Injection of [ 3H]noradrenaline (50 μ Ci) into the upper cervical spinal cord resulted in retrograde labeling of cells in the locus coeruleus, subcoeruleus and the ventrolateral pontine tegmentum, that correspond in position to the neurons of the A6, A7 and A5 catecholamine cell groups, respectively. Injection of [ 3H]serotonin (20 μ Ci) into the upper cervical spinal cord was associated with retrograde labeling of cells in the raphe pallidus, obscurus and magnus nuclei that correspond in position to those of the B1, B2 and B3 serotonin cell groups, respectively. Injection of True Blue into the upper cervical spinal cord was followed by retrograde labeling of a large number of cells located in the areas where cells were retrogradely labeled by [ 3H]choline, [ 3H]noradrenaline and [ 3H]serotonin, and additionally, in the solitary tract nucleus, the lateral, parvicellular medullary reticular formation, the caudal and oral pontine reticular formation, the mesencephalic reticular formation and the superior colliculus. These results indicate that from the cervical spinal cord, [ 3H]choline selectively retrogradely labels a certain population of non-monaminergic, acetylcholinesterase-positive cells localized in the medial medullary, and secondarily the dorsolateral pontine, reticular formation, the lateral vestibular nucleus, and the red nucleus.

Substantia nigra projection to medullary reticular formation: Relevance to oculomotor and related motor functions in the cat

Wheat germ lectin-horseradish peroxidase was injected into the medial medullary reticular formation in 4 cats. Retrogradely labeled cell bodies were observed in the ipsilateral substantia nigra pars reticulata and bilaterally in the superior colliculus, oculomotor nucleus and fastigial nucleus of the cerebellum. The nigral projection to this reticular formation area, which receives diffuse afferents from cortex, cerebellum, oculomotor system and vestibular nucleus and projects to spinal cord, may be of some relevance in understanding the nigral role in postural, head- and eye-movement control.

Selective retrograde transneuronal transport of wheat germ agglutinin-conjugated horseradish peroxidase in the oculomotor system.

The fate of wheat germ agglutinin-conjugated horseradish peroxidase (WGA/HRP) subsequent to its uptake and retrograde axonal transport in abducens motoneurons of the monkey was studied using histochemical localization of WGA/HRP reaction product and light microscopy. Injections of WGA/HRP into monkey lateral rectus muscles produced a pattern of labelled motoneurons like that obtained with native HRP. In contrast to the native HRP data, WGA/HRP injections consistently labelled additional neuronal populations in the ipsilateral medial vestibular nucleus and contralateral dorsal medullary reticular formation. These regions correspond to those containing neurons known to make inhibitory synaptic contact with abducens motoneurons. No labelled neurons were observed in regions which contain excitatory premotor neurons. These data are consistent with the notion of retrograde transneuronal transport of WGA/HRP to premotor neurons. The specificity of the transneuronal exchange is indicated by the finding that only certain populations of premotor neurons were labelled. The precise manner by which preferential transneuronal transport of WGA/HRP is attained remains to be determined.

Effect of interruption of bulbospinal pathways on lordosis, posture, and locomotion

The effect of interruption of descending bulbospinal projections arising from (i) the medial medullary reticular formation: nucleus reticularis gigantocellularis (NGC) and nucleus reticularis magnocellularis (NMC), (ii) nucleus raphe magnus, and (iii) nucleus subceruleus on the lordosis reflex, posture, and locomotion was examined. After interruption of descending NGC-NMC fibers, time-dependent deficits in lordosis were observed during the first two postoperative weeks with subsequent reflex recovery. A syndrome of postural and locomotor deficits occurred after NGC-NMC lesions. Postoperatively, postural reflexes were dominated by increased extensor tone which resulted in limb hyperextension. Specific time-dependent deficits in the control of extensor and flexor muscle groups involved in locomotion resulted in a motor syndrome that is detailed. After interruption of NMC fibers alone, no postural or locomotor deficits occurred and effects on lordosis were limited to the first few postoperative days. The NMC data suggested that the majority of the functional deficits seen after NGC plus NMC lesions were due to destruction of the NGC descending projections. Interruption of NSC bulbospinal fibers resulted in a significant depletion of spinal cord noradrenaline (NA) but not serotonin (5-HT). Functional deficits, when present, were limited to the first few postoperative days. Although interruption of descending NRM fibers resulted in a significant depletion of spinal cord 5-HT concentrations, greater depletions occurred in the NGC plus NMC group. No functional deficits were observed in the NRM group. These data suggest that interruption of NGC-NMC fibers destroyed the descending output from neurons which integrate the hormonal and sensory information necessary for lordosis.

Alpha-chloralose suppression of neuronal activity

Alpha-chloralose, an anesthetic agent widely used in neurophysiologic studies, caused a significant and long-lasting suppression of single neuron activity recorded from two areas of the central nervous system in decerebrate cats. A 50 mg/kg dose (an average anesthetic dose used in many neurophysiologic studies) caused suppression of spontaneous and evoked activity of neurons in the dorsal horn of the spinal cord and greater suppression of neurons in the nucleus reticularis gigantocellularis (NRGC) of the medial medullary reticular formation. Many researchers are of the opinion that alpha-chloralose causes less suppression of the central nervous system (CNS) than other commonly used anesthetic agents. The neuronal suppression recorded in this study appears similar in many ways to suppression caused by other anesthetic agents in the same two areas of the CNS. The results of the present study suggest that alpha-chloralose may be capable of producing significant suppression of neurons in the dorsal horn of the spinal cord and NRGC. Its ability to influence other areas of the CNS should not be inferred from these results, but the data do indicate the importance of evaluating the effects of anesthetics upon neurophysiologic systems under study.

Suppression of bulboreticular unit responses to noxious stimuli by analgesic mesencephalic stimulation.

The responses of 302 neurons in the medial medullary reticular formation (MRF) to a variety of noxious and innocuous somatic stimuli were studied in anesthetized and awake rats. In addition, the effects of analgesic electrical stimulation in the mesencephalon (MES) on unit responses were examined. Tail shock was the most effective stimulus, exciting more than 80% of all units recorded. This stimulus was considered separately during data analysis, since it could not be classified as noxious or innocuous. Noxious somatic stimuli (including pinch, firm pressure, pin prick, and radiant heating of the tail above 45 degrees C were especially effective in eliciting discharge in a significant fraction of all cells in both awake (123/205) and anesthetized (45/97) animals. Nociceptive neurons could be classified as nociceptive specific (NS) or wide dynamic range (WDR) depending on their responses to all somatic stimuli tested. Nociceptive neurons showed no preferential anatomical distribution. Most neurons, including those responsive to noxious inputs, exhibited large, often bilateral receptive fields which frequently covered the tail, one or more limbs, and extensive areas of the body or head. Electrical stimulation within or adjacent to the mesencephalic periaqueductal gray matter depressed the spontaneous and evoked discharge of MRF neurons in both acute and chronic preparations. This inhibition showed a significant preference (p less than 0.001, chi-square statistic) for units that were excited by somatic and especially noxious stimuli. No units were facilitated by MES stimulation. In the awake rat, unit suppression closely followed the time course and level of MES-induced analgesia. Excitability data from the acute experiments suggest that this response inhibition may be the result of a direct action on MRF neurons. Anesthesia severely depressed the spontaneous discharge of MRF neurons as well as the activity evoked by innocuous somatic stimulation. Our data suggest that analgesia produced by MES stimulation is at least in part due to the depression of MRF unit activity, and support the hypothesis that MRF neurons play a critical role in the mediation of behavioral responses to noxious stimuli.

Connections of brainstem respiratory nuclei studied by the retrograde horseradish peroxidase axon transport method

Intrabulbar connections of respiratory nuclei and the medullary reticular formation and also descending pathways from these structures in the spinal cord were studied by the retrograde horseradish peroxidase axonal transport method in cats. Neurons of the nucleus ambiguus and nucleus retroambigualis (ventral respiratory group) and of the ventrolateral part of the nucleus of the tractus solitarius (dorsal respiratory group) were shown to form direct two-way connections with each other and with the medial region of the medulla. Neurons of the pneumotaxic center send uncrossed axons to the nucleus ambiguus and to the medial medullary reticular formation. Neurons of the contralateral homonymous nucleus and neurons of the nucleus of the tractus solitarius are sources of projections of the locus coeruleus. A well developed system of direct connections was found between neurons of respiratory nuclei of the two halves of the brain. The possible role of these nuclear formations in genesis of the respiratory rhythm and regulation of the respiratory and other motor functions of the reticular formation is discussed.

Supraspinal and segmental input to lumbar epaxial motoneurons in the rat

Inputs to medial longissimus (ML) and lateral longissimus (LL) motoneurons were studied in urethane or urethane-chloralose anesthetized rats by recording from ML and LL nerves while stimulating ipsilateral lumbosacral dorsal roots, medial medullary reticular formation (RF), vestibular nuclei (VN), dorsal midbrain (MDBR), or ventromedial hypothalamus (VMH). Stimulation of appropriate dorsal roots produced short-latency responses (1.5–3.0 ms) in nerves to medial longissimus or lateral longissimus. The connections underlying these responses, which could be monosynaptic, are weak, since generally two or more stimuli were necessary for a response to occur. Short-latency LL nerve responses required more dorsal root stimuli than did ML nerve responses and stable LL responses sometimes could not be obtained, suggesting that segmental reflexes to a back muscle (LL) could be weaker than those to a proximal tail muscle (ML). Trains of conditioning stimuli delivered to the RF, VN, and MDBR facilitated segmental responses in ML nerves or LL nerves. Temporal profiles of facilitation of ML differed for the three regions. On one extreme, the facilitation produced by RF conditioning required few stimuli (median, 3 shocks) and peak facilitation occurred at short condition-test intervals (median, 1.5 ms). On the other extreme, facilitation produced by MDBR conditioning required long trains (median, 14 stimuli) and peak facilitation occurred at longer condition-test intervals (median, 10ms). Stimulation within the VMH never facilitated ML or LL nerve activity. These results demonstrate excitatory connections from reticular formation, vestibular nuclei and the dorsal midbrain to medial longissimus and lateral longissimus. Such connections could be involved in behaviors mediated by midbrain, and in postural regulation through brain stem control of axial musculature. Motoneuron cell bodies for LL, ML and lumbar transversospinalis (TS) muscles were localized by ejecting dye at sites where unitary antidromic responses to muscle nerve stimulation were recorded extracellularly. ML cells were found ventrolaterally in the L 6-S 1 ventral horn. LL and TS cells were found medially in the ventral horn of the lumbar enlargement.

Activity of medullary reticular formation neurons in the unrestrained cat during waking and sleep

Single units, recorded in the medial medullary reticular formation (RF) in unrestrained, behaving cats, discharged in conjunction with specific movements and postures. Most cells were also active during REM sleep. Discharge rates in active waking and REM sleep were positively correlated and discharge patterns in these states were similar. We conclude that activity in these cells is related to the motor activation occurring in both active waking and REM sleep. We found no cells whose discharge was related in a non-specific way to motor tone or to REM sleep atonia. We discuss mechanisms by which medullary units with specific motor relations may give rise when stimulated to the relatively non-specific motor effects previously reported.

Effects of ketamine on nociceptive cells in the medial medullary reticular formation of the cat.

Anatomic, physiologic and behavioral evidence suggests that the neurons in the nucleus reticularis gigantocellularis of the medial medullary reticular formation may act as a relay station for the transmission of nociceptive information from the spinal cord to higher brain centers. The nucleus reticularis gigantocellularis may also be the site of action of analgesic agents, such as ketamine hydrochloride. Utilizing extracellular microelectrodes in 23 decerebrate cats, the authors measured the effect of ketamine on neurons in the nucleus reticularis gigantocellularis that were excited by electrical stimulation of peripheral nerves. The frequency of spontaneous single-unit firing activity in the nucleus reticularis gigantocellularis was suppressed by 31 +/- 11 (mean +/- 1 SE) and by 62 +/- 7 per cent with ketamine, 1.0 and 2.5 mg/kg, iv, respectively. The frequency of evoked single-unit activity was suppressed by 57 +/- 9 and 79 +/- 5 per cent with ketamine, 1.0 and 2.5 mg/kg, respectively. Ketamine produces significant depression of single-unit activity of the cells in the nucleus reticularis gigantocellularis, suggesting that this may be an important site of its analgesic action.

Afferent and Efferent Organization of the Prepositus Hypoglossi Nucleus

This chapter provides general outline of findings regarding the afferent and efferent organization of the prepositus nucleus. From retrograde horseradish peroxidase (HRP) experiments, it appears that, qualitatively, the important inputs to this nucleus come from the vestibular nuclei, perihypoglossal nuclei, reticular formation, extraocular nuclei, accessory oculomotor nuclei and the cerebellum. The efferents of the prepositus hypoglossi were studied with autoradiographic, electrophysiologioal and both intraand extracellular HRP techniques. The prepositus appears to establish efferent connections bilaterally with the cerebellar cortex, the interposed and medial cerebellar nuclei, the vestibular nuclei, the perihypoglossal nuclei, the medial medullary and pontine reticular formation, the extraocular motor nuclei, the accessory occulomotor nuclei and the region around the parabigeminal nucleus. Intracellular injections of HRP reveal that a single prepositus neuron sends collaterals to several of the above brain stem areas. Evidence is also presented that indicates that there is a direct projection from prepositus to ocular motoneurons and that it is excitatory in effect. Such a wealth of anatomical connections suggests that the prepositus is much more than a simple site for either preoculomotor or precerebellar activity.

The participation of medullary reticular formation in clonidine-induced hypotension in rats

The possible participation of the medial medullary reticular formation (MMRF) in clonidine-induced hypotension has been investigated in lightly pentobarbital sodium (40 mg/kg, i.p.) anaesthetized rats. Bilateral focal electrolytic lesions of the MMRF resulted in an elicited hypertension without the subsequent hypotension normally observed when clonidine (10 μg/kg) was injected systemically. On the other hand, muinjection of clonidine, at an ineffective dose (0.25 μg/kg) when administered intravenously, induced a significant vasodepression with no preceding hypertension. While confirming that clonidine exerts its pressor effects by acting peripherally, this study has identified the MMRF as at least one of the central sites through which clonidine may produce depression of arterial blood pressure. The role of the MMRF has been attributed to modulation of basic circulatory reflexes. When activated by clonidine, the reticular neurones may inhibit the sympathetic discharges and facilitate the parasympathetic outflow to the cardiovascular effectors, resulting in clonidine-hypotension of central origin.

Brain stem and cerebellar lesions in female rats. I. Tests of posture and movement

Lesions were placed in a number of brain stem and cerebellar structures to determine their importance to the control of posture and movement in rats. Lesions of the lateral vestibular nucleus, superior vestibular nucleus, or inferior olive produced severe disturbances of posture and movement. Rats with such lesions had pronounced tremor, abnormal head posture and asymmetrical trunk posture, and failed in a balance beam test. Lesions of the medial medullary reticular formation and medial vestibular nucleus had less overall effect on posture and movement, while animals with lesions of the deep cerebellar nuclei were relatively unimpaired. All lesion groups showed some recovery of function during the course of postoperative testing.

The motor nucleus of the facial nerve in the opossum (Didelphis marsupialis virginiana). Its organization and connections.

AbstractThe organization of the facial nucleus was studied in the opossum by localizing neurons which stin poorly for acetylcholinesterase activity following transection of identified facial rami. The caudal auricular representation is limited to the ventromedial extreme of the nucleus, whereas the neurons contributing to the cervical ramus are situated dorsally and medially. The zygomatic representation extends throughout the intermediate portion of the nucleus, apparently overlapping with that of the palpebral and rostral auricular muscles which is limited to the ventral extreme of the intermediate zone. The buccolabial area is particularly large in the opossum and encompasses most of the lateral facial enlargement.Midbrain‐facial projections were identified from the superior colliculus, the midbrain tegmentum (particularly caudal ventromedial areas) and the red nucleus. The location of terminal degeneration in the facial nucleus following lesions within each of these areas was plotted and interpreted in light of facial organization. Of particular note is the fact that the fibers of rubral origin distribute preferentially to the zygomatic and, to some extent, buccolabial areas, whereas the ventromedial tegmental system distributes most strongly to the areas of caudal auricular, cervical, palpebral and rostral auricualar representation. The medial and intermediate regions of the facial nucleus receive a denser midbrain projection than does the lateral (buccolabial) area. In contrast, evidence was obtained for an extensive facial projection from the parvocellular reticular formation and the caudal spinal trigeminal nucleus which strongly favors the buccolabial enlargement. The possibility exists that the medial pontine and medullary reticular formation as well as portions of the dorsal column nuclei also have a facial projection. Spino‐facial fibers arise rostral to the cervical enlargement and show a predilection for the medial facial enlargement (cervical and caudal auricular areas). Although some systems distribute preferentially to specific areas of the facial nucleus, overlap is present suggesting considerable integration.

Control of the tonic vibration reflex by the brain stem reticular formation in the cat

The tonic vibration reflex (TVR) has been used to investigate the areas in the brain stem which influence tone in flexor and extensor muscles in the hind limb of the cat. Tonic contraction of triceps surae and tibialis anterior was inhibited by stimulation of the medial medullary reticular formation and facilitated by lateral medullary stimulation. Between the inhibitory and facilitatory areas, a mixed area was found which produced facilitation on direct stimulation which was altered to inhibition when the contralateral motor cortex or internal capsule was stimulated simultaneously. A small area in the caudal medulla reciprocally enhanced and suppressed the TVR in flexors and extensors respectively.

The influence of different descending systems on the tonic stretch reflex in the cat

The relation between muscle tension and length during extension at a slow constant rate, i.e., the static characteristics of the stretch reflex of the gastrocnemius and the plantaris muscles were studied in decrebrated cats. The ipsilateral lateral vestibular nucleus (Deiters') was continuously stimulated while extending the muscle, which resulted in an increase of muscle activity. The muscle was extended several times with different strength of stimulation, and a family of static characteristics related to the stimulation of the Deiters' nucleus was thus obtained. The same family was also obtained, when the stimulation of the Deiters' nucleus was combined with stimulation of other brain stem structures inhibiting extensor muscles (the pyramidal tract, the medial medullary reticular formation, the contralateral Deiters' nucleus). Similar results were also obtained with the anterior tibial muscle during stimulation of the pyramidal tract or the medial medullary reticular formation. Families of static characteristics of both extensors and flexor were preserved during selective paralysis of gamma efferents by application of novocain to the muscle nerve.